Welcome to the thermonuclear club

After North Korea’s September 2016 nuclear test, the writing was on the wall. North Korea was moving toward testing an ICBM and a thermonuclear weapon to arm it.

On Sept. 2, North Korean state media released images of Kim Jong Un standing next to what appeared to be a two-stage thermonuclear weapon small enough to arm one of North Korea’s long-range missiles that can strike the United States. And before analysts could pretend it was filled with Styrofoam peanuts, kaboom! North Korea conducted its largest nuclear explosion ever. North Korea announced that it had tested a two-stage thermonuclear weapon.

How big was the bomb? This is tricky. Seismologists measure nuclear explosions with the same scale used for earthquakes; shaking ground is pretty much all the same. The United States Geological Service, as well as its Chinese counterpart, both estimate the size of the explosion as 6.3. (The Comprehensive Nuclear Test Ban Treaty Organization has a slightly lower estimate at 5.8. CTBTO estimates are usually a bit lower.)

That’s really big. The September 2016 explosion was only a 5.2, and since the scale is logarithmic, that means this explosion is more than 10 times larger than the one in September. The explosion was so powerful that USGS recorded a second seismic event a few minutes later, which appears to have been a collapse inside the cavity created by the explosion.

There is a bit of estimation involved in calculating the size of the explosion that produced the seismic signal. It depends on the geology of the test site, how well seismic waves propagate from that site, how deeply buried in rock the explosion, and how well coupled the explosion was to the surrounding rock. The standard available equations give slightly different answers, but they suggest the yield will be around a few hundred kilotons as long as the estimate of 6.3 holds. That’s an order of magnitude larger than anything North Korea has ever exploded before and about the same yield as modern U.S. thermonuclear warheads. (The American ones are more efficient.)

These calculations will also depend on where the test occurred. Right now it looks like the test happened at Punggye-ri test site, but in which mountain? If North Korea tested in the new tunnel we discovered a few years ago, it might have been more deeply buried than other tests and therefore look smaller than it was. And if North Korea tested inside one of the cavities created by past nuclear explosions, that might also help hide how large it was. You’re going to have to be patient as seismologists, nuclear wonks and the like settle on a good answer. But even the low end of the estimates are big. Big enough that the explosion was far too large to be the compact fission device Kim Jong Un posed with back in 2016.

What sort of bomb was it? The U.S. intelligence community is calling it an “advanced nuclear device.” That’s a hedge because they are still waiting to see if the test releases any radionuclides that might give a hint as to the composition of the bomb. With only preliminary yield estimates, it’s hard to completely exclude some other thermonuclear concepts, but all of those options are still roads that lead to a thermonuclear weapon.

For now, those of us reliant on open-source data are left with analyzing the device in the pictures, which North Korea said was a two-stage thermonuclear weapon. The thing is shaped like a peanut, with each nut in the shell as one of the device’s stages. The larger nut is the fission primary, most likely based on the device tested last year, while the smaller one is an apparently spherical secondary of thermonuclear material.

I am old enough to remember when leaks appeared in the press stating that the U.S. W88 nuclear warhead was shaped like a peanut, with a spherical secondary and slightly larger primary. People freaked out that classified information like that was available in the press. I thought they were overreacting, but still, it’s a little weird to see Kim Jong Un standing next to a giant peanut of death. Kim is definitely not getting a Q clearance from DOE as long as he keeps disclosing restricted data like this.

The North Koreans are boasting, but I see no particular reason to doubt them. The resulting explosion was large enough to be a thermonuclear weapon, and six nuclear tests is plenty to develop such a device. Still, it would be nice to have some official confirmation.

I am seeing a lot of people saying: so what? A nuclear weapon is a nuclear weapon. What does it matter?

Well, obviously a larger nuclear weapon does more damage. Go ahead and check out Alex Wellerstein’s Nuke Map. Drop a 30-kiloton bomb on Trump Tower, then drop a 300-kiloton bomb there. Larger yields help compensate for less accurate missiles. If your goal is to consume Manhattan in a cleansing thermonuclear firestorm with missiles that have accuracies on the order of a kilometer or so, a couple hundred kilotons is going to be a lot more credible of a threat.

The North Koreans also went out of their way to taunt us about electromagnetic pulse (EMP) effects, I suppose because they think we’re worried about them. I think it’s laughable to imagine that North Korea would waste a nuclear weapon hoping to knock down parts of the power grid. I would much prefer the North Koreans waste nuclear weapons trying to achieve an uncertain EMP effect than incinerating cities with real people pushing strollers with real babies.

But there is also a deeper meaning here. We have struggled, over and over again, to accept North Korea’s stated goal of possessing a thermonuclear weapon that can be delivered against targets in the United States. The North Koreans spent all summer talking about how its new missiles were designed to carry a “large sized heavy nuclear weapon.” But when I told people that meant a thermonuclear weapon, a lot of them laughed. We’ve gotten comfortable with the idea that wars are things that happen in other places, that we can take out tinpot dictators with little or no risk to ourselves. The idea that the North Koreans could retaliate, that they could threaten us here in the United States, is something that U.S. officials have openly described as “unimaginable.”

The thing is, you don’t have to imagine it, at least not any more. It’s right there in front you: the missile launches on the Fourth of July, the pictures of Kim Jong Un smiling with his the bomb, and now a nice loud bang.

Jeffrey Lewis is director of the East Asia Nonproliferation Program for the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies at Monterey.

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